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SubjectRe: [PATCH 00/13] convert block layer to bioset_init()/mempool_init()
On Mon, May 21 2018 at 11:09am -0400,
Jens Axboe <axboe@kernel.dk> wrote:

> On 5/21/18 9:04 AM, Mike Snitzer wrote:
> > On Mon, May 21 2018 at 10:52am -0400,
> > Jens Axboe <axboe@kernel.dk> wrote:
> >
> >> On 5/21/18 8:47 AM, Mike Snitzer wrote:
> >>> On Mon, May 21 2018 at 10:36am -0400,
> >>> Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>>> On 5/21/18 8:31 AM, Mike Snitzer wrote:
> >>>>> On Mon, May 21 2018 at 10:19am -0400,
> >>>>> Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>
> >>>>>> On 5/21/18 8:03 AM, Mike Snitzer wrote:
> >>>>>>> On Sun, May 20 2018 at 6:25pm -0400,
> >>>>>>> Kent Overstreet <kent.overstreet@gmail.com> wrote:
> >>>>>>>
> >>>>>>>> Jens - this series does the rest of the conversions that Christoph wanted, and
> >>>>>>>> drops bioset_create().
> >>>>>>>>
> >>>>>>>> Only lightly tested, but the changes are pretty mechanical. Based on your
> >>>>>>>> for-next tree.
> >>>>>>>
> >>>>>>> By switching 'mempool_t *' to 'mempool_t' and 'bio_set *' to 'bio_set'
> >>>>>>> you've altered the alignment of members in data structures. So I'll
> >>>>>>> need to audit all the data structures you've modified in DM.
> >>>>>>>
> >>>>>>> Could we get the backstory on _why_ you're making this change?
> >>>>>>> Would go a long way to helping me appreciate why this is a good use of
> >>>>>>> anyone's time.
> >>>>>>
> >>>>>> Yeah, it's in the first series, it gets rid of a pointer indirection.
> >>>>>
> >>>>> "Allows mempools to be embedded in other structs, getting rid of a
> >>>>> pointer indirection from allocation fastpaths."
> >>>>>
> >>>>> So this is about using contiguous memory or avoiding partial allocation
> >>>>> failure? Or both?
> >>>>>
> >>>>> Or more to it? Just trying to fully appreciate the theory behind the
> >>>>> perceived associated benefit.
> >>>>
> >>>> It's about avoiding a pointer indirection. Instead of having to
> >>>> follow a pointer to get to that struct, it's simple offset math off
> >>>> your main structure.
> >>>>
> >>>>> I do think the increased risk of these embedded bio_set and mempool_t
> >>>>> themselves crossing cachelines, or struct members that follow them doing
> >>>>> so, really detracts from these types of changes.
> >>>>
> >>>> Definitely something to look out for, though most of them should be
> >>>> per-dev structures and not in-flight structures. That makes it a bit
> >>>> less sensitive. But can't hurt to audit the layouts and adjust if
> >>>> necessary. This is why it's posted for review :-)
> >>>
> >>> This isn't something that is easily caught upfront. Yes we can all be
> >>> busy little beavers with pahole to audit alignment. But chances are
> >>> most people won't do it.
> >>>
> >>> Reality is there is potential for a regression due to false sharing to
> >>> creep in if a hot struct member suddenly starts straddling a cacheline.
> >>> That type of NUMA performance killer is pretty insidious and somewhat
> >>> tedious to hunt down even when looking for it with specialized tools:
> >>> https://joemario.github.io/blog/2016/09/01/c2c-blog/
> >>
> >> IMHO you're making a big deal out of something that should not be.
> >
> > I raised an issue that had seemingly not been considered at all. Not
> > making a big deal. Raising it for others' benefit.
> >
> >> If the dm bits are that sensitive and cache line honed to perfection
> >> already due to previous regressions in that area, then it might
> >> not be a bad idea to have some compile checks for false cacheline
> >> sharing between sensitive members, or spilling of a sub-struct
> >> into multiple cachelines.
> >>
> >> It's not like this was pushed behind your back. It's posted for
> >> review. It's quite possible the net change is a win for dm. Let's
> >> focus on getting it reviewed, rather than pontificate on what
> >> could potentially go all wrong with this.
> >
> > Why are you making this personal? Or purely about DM? I'm merely
> > pointing out this change isn't something that can be given a quick
> > blanket "looks good".
>
> I'm not making this personal at all?! You raised a (valid) concern,
> I'm merely stating why I don't think it's a high risk issue. I'm
> assuming your worry is related to dm, as those are the reports
> that would ultimately land on your desk.

Then we'll just agree to disagree with what this implies: "It's not like
this was pushed behind your back."

Reality is I'm fine with the change. Just think there is follow-on work
(now or later) that is needed.

Enough said.

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